Wafer polishing control system for chemical mechanical planarization machines

ABSTRACT

Methods and apparatus for applying a uniform polishing pressure on a wafer are disclosed. According to one aspect of the present invention, a chemical mechanical planarization polishing apparatus includes a polishing pad, a wafer holder, and a force control system. The wafer holder supports a wafer to be polished using the polishing pad. The polishing pad is arranged to move relative to the wafer holder such that an area of contact between the wafer holder and the polishing pad varies. The force control system including a controller and a plurality of actuators that apply forces to the polishing pad. The controller controls the forces as the area of contact varies to substantially maintain a first polishing pressure on the wafer arranged to be supported by the wafer holder.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to semiconductor processingequipment. More particularly, the present invention relates to arelatively compact wafer polishing apparatus which is capable ofmaintaining substantially uniform contact pressure on a wafer polishingarea during operation.

2. Description of the Related Art

Chemical mechanical planarization apparatuses are generally used duringsemiconductor fabrication processes to polish wafer surfaces. As will beappreciated by those skilled in the art, chemical mechanicalplanarization is an abrasive process that polishes a wafer to create asmooth surface through the use of a chemical slurry and circular motionsof a polishing pad and a wafer. A smooth or even surface on a wafer iscritical to ensure the integrity of a semiconductor formed using thewafer, e.g., to ensure that interconnects between layers of the waferare not deformed and to ensure that desired photolithographic depths offocus are maintained.

FIG. 1 a is a diagrammatic top-view representation of one conventionalchemical mechanical planarization apparatus used for wafer polishing. Anapparatus 102 includes a polishing pad 104 which has a significantlylarger diameter than the diameter of a wafer 108. Apparatus 102 alsoincludes a swinging arm 106 that allows polishing pad 104 to be movedrelative to wafer 108, which is typically held in a wafer chuck (notshown) that, like polishing pad 104, spins while apparatus 102 is inuse.

The use of polishing pad 104 that is larger than wafer 108 generallyensures that substantially even polishing of wafer 108 occurs, as arelatively even contact pressure may be readily maintained betweenpolishing pad 104 and wafer 108. Hence, a surface of wafer 108 may berelatively evenly polished. However, when polishing pad 104 issignificantly larger than wafer 108, apparatus 102 may be inconvenientand, hence, impractical to use. For example, the overall footprint ofapparatus 102 may be larger than desired, and power requirementsassociated with rotating polishing pad 104 relatively to wafer 108 maybe higher than desired. In addition, the cost of a polishing pad 104that is larger than a wafer 108 may be relatively high.

Some systems use a polishing pad that has a smaller diameter than awafer being polished. FIG. 1 b is a diagrammatic top-view representationof a wafer polishing apparatus which includes a polishing pad which issmaller than a wafer being polished. An apparatus 112 includes apolishing pad 114 which is arranged to polish a surface of a wafer 118.When substantially all of a polishing surface of polishing pad 114 is incontact with a surface of wafer 118, as shown, a first contact pressuremay be maintained between polishing pad 114 and wafer 118. However, whenat least a part of a polishing pad 114 is not in contact with a surfaceof wafer 118, as shown in FIG. 1 c, a contact pressure between polishingpad 114 and wafer 118 is not the same as the first contact pressurewhich may be maintained when substantially all of a polishing surface ofpolishing pad 114 is in contact with wafer 118. Specifically, whenpolishing pad 114 has a smaller diameter than the diameter of wafer 118,the application of a constant force to polishing pad 114 does not enablea uniform contact pressure to be maintained irregardless of the positionof polishing pad 114 relative to wafer 118, as the contact pressurevaries depending upon how much of polishing pad 114 is in contact withwafer 118. As a result, a polishing process which involves apparatus 118generally does not allow for a surface of wafer 118 to be evenlypolished.

The inability to enable relatively even polishing of a surface of awafer to occur unless a wafer-polishing pad has a diameter that issignificantly larger than the diameter is often problematic. Often,trade-offs may have to be made between the higher costs associated withan apparatus which enables relatively even polishing of a surface andthe lower costs associated with an apparatus which provides for lesseven polishing.

Therefore, what is needed is a relatively compact and cost-efficientapparatus which allows for relatively even polishing of a wafer surface.That is, what is desired is a chemical mechanical planarizationpolishing apparatus which enables a wafer-polishing pad that is notsignificantly larger than a wafer to provide relatively even polishingof a surface of the wafer.

SUMMARY OF THE INVENTION

The present invention relates to a chemical mechanical planarizationpolishing apparatus which allows a substantially uniform polishingpressure to be maintained on the wafer. According to one aspect of thepresent invention, a chemical mechanical planarization polishingapparatus includes a polishing pad, a wafer holder, and a force controlsystem. The wafer holder supports a wafer to be polished using thepolishing pad. The polishing pad is arranged to move relative to thewafer holder such that an area of contact between the wafer holder andthe polishing pad varies. The force control system including acontroller and a plurality of actuators that apply forces to thepolishing pad. The controller controls the forces as the area of contactvaries to substantially maintain a first polishing pressure on the waferarranged to be supported by the wafer holder.

In one embodiment, the controller is arranged to vary the forces as thearea of contact varies to substantially maintain the first polishingpressure on the wafer arranged to be supported by the wafer holder. Inanother embodiment, the controller determines the forces based upon aposition associated with the polishing pad, the first polishingpressure, an air pressure load on the polishing pad, and a distancebetween a center of the polishing pad and a center of gravity associatedwith the chemical mechanical planarization apparatus.

A chemical mechanical planarization polishing apparatus which includes aforce control system that allows the magnitude of forces applied on apolishing pad to be adjusted as needed enables a substantially uniformpolishing pressure to be maintained on a wafer that is being polishedusing the apparatus. By allowing a desired polishing pressure to bemaintained regardless of how large a contact area between the polishingpad and the wafer is, i.e., by adjusting forces applied by actuators ofthe force control system based upon the size of a contact area betweenthe polishing surface of the polishing pad and the polishing surface ofthe wafer, the likelihood that the integrity of the polished wafer iscompromised during the polishing process may be reduced.

According to another aspect of the present invention, a method forplanarizing a surface of a wafer using an apparatus which includes aforce system with a plurality of actuators, a polishing pad, and a chuckarranged to support the wafer substantially in contact with thepolishing pad involves polishing the wafer using the polishing pad.Polishing the wafer using the polishing pad includes rotating the waferwhile the wafer is in contact with the polishing pad. The method alsoincludes determining a current area of contact between the polishing padand the wafer, and adjusting the forces applied by each of the pluralityof actuators to substantially maintain a first polishing pressure on thewafer. The magnitudes of the forces are adjusted based upon the currentarea of contact.

In one embodiment, the method also includes determining the forces to beapplied by each of the plurality of actuators to substantially maintainthe first polishing pressure on the wafer. Determining the forcesincludes determining a current position associated with the polishingpad, identifying the first polishing pressure, identifying an airpressure load on the polishing pad, and determining a current distancebetween a center of the polishing pad and a center of gravity associatedwith the chemical mechanical planarization apparatus.

These and other advantages of the present invention will become apparentupon reading the following detailed descriptions and studying thevarious figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 a is a diagrammatic top-view representation of one conventionalchemical mechanical planarization apparatus used for wafer polishing.

FIG. 1 b is a diagrammatic top-view representation of a wafer polishingapparatus which includes a polishing pad which is smaller than a waferbeing polished.

FIG. 1 c is a diagrammatic top-view representation of a wafer polishingapparatus which includes a polishing pad which is smaller than a waferbeing polished and is not completely in contact with the wafer.

FIG. 2 is a diagrammatic top-view representation of a chemicalmechanical planarization polishing apparatus in accordance with anembodiment of the present invention.

FIG. 3 is a block diagram representation of a polishing apparatus with anon-contact force system in accordance with an embodiment of the presentinvention.

FIG. 4 is a diagrammatic representation of an orientation of actuatorsof a force system in accordance with an embodiment of the presentinvention.

FIG. 5 is a block diagram representation of a wafer polishing controlsystem which is suitable for controlling a wafer polishing apparatus inaccordance with an embodiment of the present invention.

FIGS. 6 a and 6 b are a process flow diagram which illustrates a methodof performing chemical mechanical planarization polishing on a wafer inaccordance with an embodiment of the present invention.

FIG. 7 is a diagrammatic representation of a wafer polishing forcedistribution in accordance with an embodiment of the present invention.

FIG. 8 is a signal flow chart for a force control system module inaccordance with an embodiment of the present invention.

FIG. 9 is a diagrammatic representation of a photolithography apparatusin accordance with an embodiment of the present invention.

FIG. 10 is a process flow diagram which illustrates the steps associatedwith fabricating a semiconductor device in accordance with an embodimentof the present invention.

FIG. 11 is a process flow diagram which illustrates the steps associatedwith processing a wafer, i.e., step 1304 of FIG. 10, in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The careful control of chemical mechanical planarization machines usedfor wafer polishing is crucial in ensuring that the integrity of a waferpolished using a chemical mechanical planarization machine is notsignificantly compromised. When a chemical mechanical planarizationmachine includes a polishing pad which has a smaller diameter than awafer that is to be polished, when the polishing pressure applied by thepolishing pad on the wafer is not accurately controlled, the polishedsurface of the wafer may be relatively uneven, which often compromisesthe integrity of semiconductor devices formed using the wafer.

A force control system which includes a plurality of actuators that arearranged to apply different magnitudes of forces to sections of apolishing pad may be included as a part of a chemical mechanicalplanarization apparatus. Such a force control system allows themagnitude of forces applied on a polishing pad to be adjusted as neededto maintain a substantially uniform polishing pressure. The ability toadjust forces applied to different sections or areas of a polishing padenables a substantially uniform polishing pressure to be effectivelymaintained irregardless of whether substantially all of the polishingsurface of the polishing pad or only part of the polishing surface ofthe polishing pad are in contact with the polishing surface of a wafer.That is, the polishing pressure may be maintained at a desired level byadjusting forces applied by actuators of the force control system basedupon the size of a contact area between the polishing surface of thepolishing pad and the polishing surface of the wafer.

FIG. 2 is a diagrammatic top-view representation of a chemicalmechanical planarization polishing apparatus which includes a forcecontrol system in accordance with an embodiment of the presentinvention. A polishing apparatus 200 includes a polishing pad 204 and awafer chuck 214 which holds a wafer 208 such that wafer 208 may comeinto contact with polishing pad 204 during a wafer polishing orplanarization process. Apparatus 200 also includes an arm 218 which iscoupled to an air pressure load system 226 and a force control system ora force system 224, e.g., an electromagnetic force system, that isarranged to control the amount of force applied to polishing pad 204based on instructions provided by a controller 222. The forces appliedto polishing pad 204 using force system 224 may be suction forces whichallows portions of polishing pad 204 to effectively be pulled up.

During wafer polishing, polishing pad 204 is moving and wafer 208 isspinning or rotating, and polishing pad 204 contacts wafer 208 with adesired polishing pressure that is provided by a head base weightassociated with a polishing head (not shown) which supports polishingpad 204 and air pressure load system 226. Force system 224 is arrangedto substantially cooperate with air-pressure load system 226 to balancethe polishing head (not shown) and to maintain a substantially uniformcontact pressure on a polishing surface of wafer 208. In one embodiment,forces associated with actuators in force system 224 are controlled toeffectively compensate for the air pressure load provided by airpressure load system 226.

In general, actuators which are a part of force system 224 may besubstantially any suitable actuators. Suitable actuators include, butare not limited to, EI-core actuators, CI-core actuators, and variousother electromagnetic actuators. Actuators which are a part of forcesystem 224 may be individually controlled to effectively dynamicallycompensate for an air load such that relatively even polishing may occuron a surface of wafer 208 irregardless of how polishing pad 204 ispositioned relative to wafer 208. By way of example, force system 224may be controlled by controller 222 such that a surface of wafer 208 maybe evenly polished whether a part of a polishing surface of polishingpad 204 is not in contact with the surface of wafer 208 or whethersubstantially all of the polishing surface of polishing pad 204 is incontact with the surface of wafer 208.

Force system 224 is generally a non-contact force system. By way ofexample, when force system 224 includes actuators that are EI-coreactuators, E-cores of the actuators may be coupled to force system 224while I-cores of the actuators are coupled to polishing pad 204 or anarrangement which supports polishing pad 204. Force system 224 operatesby providing an attraction force to a polishing pad arrangement whichincludes polishing pad 204, e.g., such that an E-core attracts anI-core. The attraction force pulls up on, e.g., effectively appliessuction to, polishing pad 204 as appropriate to compensate for the airload. FIG. 3 is a block diagram representation of a polishing apparatuswith a non-contact force system in accordance with an embodiment of thepresent invention. A polishing apparatus 300 includes a rotating wafer308 which is arranged substantially beneath a moving polishing padarrangement 305 which is suitable for polishing, e.g., abrading, a topsurface of the rotating wafer. A fixed, non-contact force system 324 isarranged over rotating polishing pad arrangement 305 such that forcesystem 324 may apply magnetic forces that effectively pull up onportions of a polishing pad of rotating polishing pad arrangement 305.By pulling up portions of the polishing pad, the overall polishingpressure associated with polishing a surface of rotating wafer 308 maybe maintained at a constant level such that even polishing may occurirregardless of whether all of or only a portion of a polishing surfaceof a rotating polishing pad comes into contact with rotating wafer 308.

When force system 324 includes E-cores of EI-core actuators, androtating polishing pad arrangement 305 includes I-cores of the EI-coreactuators, gap distances between each E-core and each I-core may bechanged to substantially directly affect the amount of attraction forceeffectively applied to portions of a polishing pad. Hence, E-cores offorce system 324 may be used to control the overall polishing forceassociated with rotating polishing pad arrangement 305. As will beappreciated by those skilled in the art, varying the current provided toa coil of an E-core allows the attraction force between the E-core and acorresponding I-core to be changed or otherwise controlled. In oneembodiment, multiple E-cores may essentially be associated with a singleI-core of a ring-shaped configuration, as described in co-pending U.S.patent application Ser. No. 10/430,598, filed May 5, 2003, which isincorporated herein by its entirety.

A force system such as force system 324 may include any number ofactuators. For example, a force system may include three actuators thatare positioned such that a first actuator is located substantially overthe inner part of a polishing pad which is the closest point to thewafer center, and second and third actuators are located substantiallyopposite from the first actuator, i.e., at slight offsets fromapproximately 180 degrees away from the first actuator. As shown in FIG.4, a first actuator 424 a of a force system 422 may be locatedsubstantially opposite, i.e., 180 degrees from, a center point 430between a second actuator 424 b and a third actuator 424 c. Secondactuator 424 b and third actuator 424 c are generally positioned at aslight offset from a centerline 434 of force system 422 which passesthrough first actuator 424 a.

In the described embodiment, actuators 424 b, 424 c are effectivelycontrolled together to apply sufficient force to pull up an edge of apolishing pad when the polishing surface associated with the edge of thepolishing pad is not in contact with a wafer being polished. It shouldbe appreciated, however, that actuators 424 b, 424 c may also becontrolled separately. In addition, the location of actuators 424 b, 424c may vary.

The amount of force applied using actuators 424 is dependent upon theoverall location of a polishing pad which is subjected to the forceapplied using actuators 424. Hence, given a desired polishing pressure,a controller, e.g., controller 222 of FIG. 2, may determine forces to beapplied using actuators 424 as a function of a location of the polishingpad.

FIG. 5 is a block diagram representation of a wafer polishing controlsystem which is suitable for controlling a wafer polishing apparatussuch as polishing apparatus 200 of FIG. 2 in accordance with anembodiment of the present invention. A chemical mechanical planarizationhost computer 502, which may be substantially any suitable computingsystem which includes a processor for processing command instructions,is arranged to provide arm movement control 506 for a swinging arm.Controlling arm movement allows a current polishing pad position to bedetermined, and provided to an electromagnetic force control system 510or, more generally, a force system which is used to effectively controla polishing pressure between the polishing pad and a wafer beingpolished. The amount of force generated by each actuator included inelectromagnetic force control system 510 is dependent upon a currentpolishing pad position.

In addition to a current pad position, electromagnetic force controlsystem 510 also receives information relating to a fixed air load forceand a desired polishing pressure from host computer 502. During waferpolishing, the rotating polishing pad touches the surface of the waferto be polished at a desired polishing pressure, which may be determinedby a head base weight and air air-pressure load system. Electromagneticforce control system 510 is arranged to substantially compensate for anoverloaded air-pressure force to balance the polishing head whichsupports the polishing pad, and to maintain substantially uniformcontact pressure on the polishing surface of the wafer.

Host computer 502 is also arranged to provide an air pressure loadcontrol system 514 with instructions, and to provide instructions to apad and chuck rotation controller 518. That is, host computer 502 allowsan air-pressure load to be controlled, and also allows rotation of botha polishing pad and a wafer chuck which supports a wafer to becontrolled.

With reference to FIGS. 6 a and 6 b, the steps associated with onemethod of performing chemical mechanical planarization polishing on awafer will be described in accordance with an embodiment of the presentinvention. A process 600 of performing chemical mechanical planarizationpolishing begins at step 604 in which a wafer that is to be polished ismoved to, or otherwise positioned in, a wafer chuck of a polishingapparatus. Then, in step 608, various parameters associated with apolishing process are set. By way of example, a polishing pressure, apolishing time, a polishing pad rotation speed, a wafer chuck rotationspeed, and an arm moving trajectory may be set.

Once parameters associated with the polishing process are set, anair-pressure load is set in step 612. After a predetermined amount oftime, during which air may effectively be pumped to the polishing pad, adetermination is then made in step 616 as to whether an air-load setpoint has been reached. If it is determined that the air-load set pointhas not been reached, then process flow returns to step 612 in which anair-pressure load is set.

Alternatively, if it is determined in step 616 that the air-load setpoint has been reached, then process flow proceeds to step 620 in whichthe polishing pad and the wafer chuck begin spinning. In step 624, thepolishing head or, more specifically, the polishing pad, is effectivelylowered to come into contact with the wafer supported on the waferchuck. After lowering the polishing pad, actuators of a force system areturned on in step 628. In the described embodiment, the actuators areelectromagnetic actuators, and turning on the actuators may includeproviding current to coils associated with the electromagneticactuators.

Upon turning on the actuators, the actuators begin to ramp up to setforces in step 632 which are appropriate to achieve the polishingpressure set in step 608. A determination is made in step 636 regardingwhether the set forces have been reached. It should be appreciated thatsuch a determination may be made after a predetermined amount of timehas elapsed. If it is determined that the set forces have not beenreached, process flow returns to step 632 in which the actuatorscontinue to ramp up to set forces. Alternatively, if it is determinedthat set forces have been reached, the arm moves, and polishing occurswhile actuator forces change 640. As previously mentioned, the actuatorforces change to provide a uniform polishing pressure irregardless ofwhether substantially all of a polishing surface of a polishing pad isin contact with a wafer, or only a portion of the polishing surface ofthe polishing pad is in contact with the wafer.

In step 644, it is determined if there is an unbalanced force associatedwith the polishing apparatus. If it is determined that there is anunbalanced force, then the polishing process is aborted. If it isdetermined that there is no unbalanced force, a determination is made instep 648 as to whether the polishing time set in step 608 has beenreached. When it is determined that the polishing time has not beenreached, process flow returns to step 640 in which polishing continuesto occur while actuator forces change as appropriate.

If the determination in step 648 is that the polishing time has beenreached, then the actuators are turned off in step 652, and thepolishing head is lifted in step 656. Once the polishing head is lifted,the spinning of the polishing pad and the wafer chuck is stopped in step660. Finally, the wafer is removed from the chuck in step 664, and theprocess of performing chemical mechanical planarization polishing iscompleted.

In order to determine actuator output forces needed to maintain adesired polishing pressure irregardless of a polishing pad location, theforce distribution associated with a polishing apparatus that includes aforce system with actuators may be studied. A command force vector F,which includes output forces for actuators, may be determined to be afunction of an air pressure load and base weight, a resistant force froma wafer, and the contact area between a polishing pad and the wafer.

FIG. 7 is a diagrammatic representation of a wafer polishing forcedistribution in accordance with an embodiment of the present invention.A polishing pad 704 and a wafer 708 have a contact area ‘A’ 746 which isa function of a position of a center of pressure 722 of polishing pad704 relative to a center of wafer 748. That is, contact area ‘A’ 746 isa function of a distance ‘x’ 750 between center of pressure 722 andcenter of wafer 748, i.e., contact area ‘A’ 746 is effectively contactarea ‘A(x)’ 746. It should be appreciated that contact area ‘A(x)’ 746may be calculated such that for every distance ‘x’ 750, or pad position,the corresponding contact area ‘A(x)’ 746 is known before a polishingapparatus which includes polishing pad 704 and wafer 708 is put intouse.

An air load ‘L’ 760 is effectively applied to polishing pad 704 throughcenter of pressure 722. Air load ‘L’ 760 is an air pressure load, whichis a passive load. Actuators 718 which are part of a force system applyforces ‘F₂’ 758 at a distance ‘r₂’ 764 from center of pressure 722. Anactuator 714 applies a force ‘F₁’ 754 at a distance ‘r₁’ 762 from centerof pressure 722. Force ‘F₁’ 754 and forces ‘F_(2’) 758 are arranged toenable a substantially uniform or desired polishing contact pressure ‘P’to be maintained on wafer 708. Hence, force ‘F₁’ 754 and forces ‘F₂’ 758are adjusted depending upon contact area ‘A(x)’ 746. In the describedembodiment, actuator 718 a and actuator 718 b are arranged to provideforces ‘F₂’ 758 of substantially the same magnitude and are, hence,effectively controlled together.

A resistant force ‘R’ 774 is a function of distance ‘x’ 750, and is aresistant force from wafer 708. A gravity center distance ‘g’ 770 variesdepending on distance ‘x’ 750, and expresses a distance between a pointwhere resistant force ‘R’ 774 acts and center of pressure 722. That is,gravity center distance ‘g’ 770 at a point in time is a distance betweencenter of pressure 722 and a center of gravity associated with pad 704and wafer 708 at the point in time. Gravity center distance ‘g’ 770 maybe calculated in real-time as a function of distance ‘x’ 750, i.e.,while a polishing apparatus which includes polishing pad 704 is in use,using geometric relationships, or may be determined while the polishingapparatus is in use through the use of a pre-calculated look-up tablewhich lists gravity center distances relative to pad positions.

Resistant force ‘R’ 774 is generally dependent upon a desired polishingpressure ‘P’ and contact area ‘A’ 746, as well as distance ‘x’ 750.Hence, resistant force ‘R’ 774 may be expressed as follows:R(x)=P·A(x)As previously mentioned, contact area ‘A’ 746 may be predetermined, orcalculated prior to using polishing pad 704, as a function of padposition or distance ‘x’ 750. It should be appreciated by those skilledin the art that geometric relationships may be used to calculate contactarea ‘A’ 746 as a function of distance ‘x’ 750. Polishing pressure ‘P’is a desired polishing pressure which is to be maintained while wafer708 is being polished using polishing pad 704.

Balancing forces and moments on pad 704 and wafer 770 yields thefollowing equations:F ₁+2·F ₂ +R(x)=LF ₁ ·r ₁ +R(x)·g(x)=2·F ₂ ·r ₂Rewritten in matrix form, the above equations may be expressed as:${\begin{bmatrix}1 & 2 \\r_{1} & {{- 2}r_{2}}\end{bmatrix}\begin{bmatrix}F_{1} \\F_{2}\end{bmatrix}} = \begin{bmatrix}{L - {R(x)}} \\{{- {R(x)}} \cdot {g(x)}}\end{bmatrix}$Substituting for resistant force ‘R’ 774 yields: ${\begin{bmatrix}1 & 2 \\r_{1} & {{- 2}r_{2}}\end{bmatrix}\begin{bmatrix}F_{1} \\F_{2}\end{bmatrix}} = \begin{bmatrix}{L - {P \cdot {A(x)}}} \\{{- P} \cdot {A(x)} \cdot {g(x)}}\end{bmatrix}$

A force command vector {circumflex over (F)} which expresses desiredforces F₁ 754 and F₂ 758, to be produced by actuators 714, 718,respectively, may be given as: $\overset{)}{F} = {\begin{bmatrix}F_{1} \\F_{2}\end{bmatrix} = {\begin{bmatrix}1 & 2 \\r_{1} & {{- 2}r_{2}}\end{bmatrix}^{- 1} \cdot \begin{bmatrix}{L - {P \cdot {A(x)}}} \\{{- P} \cdot {A(x)} \cdot {g(x)}}\end{bmatrix}}}$Since distance ‘r₁’ 762, distance ‘r₂’ 764, air-pressure load force andbase weight ‘L’ 760, and desired pressure ‘P’ are known, e.g., given bya user or operator of an overall chemical mechanical planarizationpolishing apparatus, force command vector {circumflex over (F)} may bedetermined given distance ‘x’ 750, as contact area ‘A’ 746 and gravitycenter distance ‘g’ 770 are effectively known if distance ‘x’ 750 isgiven.

Referring next to FIG. 8, a signal flow chart for a force control systemmodule will be described in accordance with an embodiment of the presentinvention. Within a force control system 800, a pad position 850, whichis a function of time since a polishing pad moves relative to a waferduring polishing, is provided to geometric equations 890. Through theuse of geometric equations or relationships 890, pad position 850 may beused to determine a contact area and a gravity center distance 872 asfunctions of pad position 850 and, hence, time. Contact area and gravitycenter distance 872 are then provided to a transform matrix 892, alongwith an air-pressure load 860 and a desired polishing pressure 866, toenable a force command vector {circumflex over (F)} 856, or desiredforces, to be determined. That is, given desired polishing pressure 866,preset air-pressure load 860, pad position 850 or a pad positiontrajectory from an arm encoder signal, force command vector {circumflexover (F)} 856 may be determined for all actuators associated with forcecontrol system 800. In the described embodiment, force control system800 has three associated electromagnetic actuators, so force commandvector {circumflex over (F)} 856 may include three forces. When at leasttwo of the electromagnetic actuators are controlled together, then forcecommand vector {circumflex over (F)} 856 may include a force to beproduced by a first electromagnetic actuator and a force to be producedby each of the electromagnetic actuators that are controlled together.

Force command vector {circumflex over (F)} 856 is provided as input to afeedback control system 894 which, through the use of sensors, as forexample load cell force sensors, provides an actual force output 896 foreach actuator that is sent as a feedback signal to feedback controlsystem 894. It should be appreciated that actual force output 896 foreach actuator is the force generated by each actuator, and is ideallysubstantially equal to forces specified in force command vector{circumflex over (F)} 856.

In one embodiment, when force control system 800 reads encoder countsand converts the encoder counts to pad position 850 at substantiallyevery servo sampling time, force control system 800 may update a desiredcompensation force trajectory, i.e., by updating force command vector{circumflex over (F)} 856. The new computed compensation forcetrajectory may be provided to each actuator associated with forcecontrol system 800 in order for each actuator to generate asubstantially desired actual output force 896.

Generally, the configuration of feedback control system 894 may varywidely. Typically, feedback control system 894 includes an adaptive gainadjustment servomechanism which uses a real-time force gain estimatescheme and an extra adaptive gain adjustment block in a servo loop. Onesuitable feedback control system is described in co-pending U.S. patentapplication Ser. No. 10/430,598, which has been incorporated byreference in its entirety.

With reference to FIG. 9, a photolithography apparatus which be used toas a part of an overall semiconductor fabrication apparatus that alsoincludes a chemical mechanical planarization polishing apparatus. Aphotolithography apparatus (exposure apparatus) 40 includes a waferpositioning stage 52 that may be driven by a planar motor (not shown),as well as a wafer table 51 that is magnetically coupled to waferpositioning stage 52 by utilizing an EI-core actuator, e.g., an EI-coreactuator with a top coil and a bottom coil which are substantiallyindependently controlled. The planar motor which drives waferpositioning stage 52 generally uses an electromagnetic force generatedby magnets and corresponding armature coils arranged in two dimensions.A wafer 64 is held in place on a wafer holder or chuck 74 which iscoupled to wafer table 51. Wafer positioning stage 52 is arranged tomove in multiple degrees of freedom, e.g., in up to six degrees offreedom, under the control of a control unit 60 and a system controller62. The movement of wafer positioning stage 52 allows wafer 64 to bepositioned at a desired position and orientation relative to aprojection optical system 46.

Wafer table 51 may be levitated in a z-direction 10 b by any number ofvoice coil motors (not shown), e.g., three voice coil motors. In onedescribed embodiment, at least three magnetic bearings (not shown)couple and move wafer table 51 along a y-axis 10 a. The motor array ofwafer positioning stage 52 is typically supported by a base 70. Base 70is supported to a ground via isolators 54. Reaction forces generated bymotion of wafer stage 52 may be mechanically released to a groundsurface through a frame 66. One suitable frame 66 is described in JP Hei8-166475 and U.S. Pat. No. 5,528,118, which are each herein incorporatedby reference in their entireties.

An illumination system 42 is supported by a frame 72. Frame 72 issupported to the ground via isolators 54. Illumination system 42includes an illumination source, which may provide a beam of EUV lightthat may be reflected off of a reticle. In one embodiment, illuminationsystem 42 may be arranged to project a radiant energy, e.g., light,through a mask pattern on a reticle 68 that is supported by and scannedusing a reticle stage 44 which includes a coarse stage and a fine stage.It should be appreciated that for such an embodiment, photolithographyapparatus 40 may be a part of a system other than an EUV lithographysystem. In general, a stage with isolated actuators may be used as apart of substantially any suitable photolithography apparatus, and isnot limited to being used as a part of an EUV lithography system. Theradiant energy is focused through projection optical system 46, which issupported on a projection optics frame 50 and may be supported theground through isolators 54. Suitable isolators 54 include thosedescribed in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, which are eachincorporated herein by reference in their entireties.

A first interferometer 56 is supported on projection optics frame 50,and functions to detect the position of wafer table 51. Interferometer56 outputs information on the position of wafer table 51 to systemcontroller 62. In one embodiment, wafer table 51 has a force damperwhich reduces vibrations associated with wafer table 51 such thatinterferometer 56 may accurately detect the position of wafer table 51.A second interferometer 58 is supported on projection optical system 46,and detects the position of reticle stage 44 which supports a reticle68. Interferometer 58 also outputs position information to systemcontroller 62.

It should be appreciated that there are a number of different types ofphotolithographic apparatuses or devices. For example, photolithographyapparatus 40, or an exposure apparatus, may be used as a scanning typephotolithography system which exposes the pattern from reticle 68 ontowafer 64 with reticle 68 and wafer 64 moving substantiallysynchronously. In a scanning type lithographic device, reticle 68 ismoved perpendicularly with respect to an optical axis of a lens assembly(projection optical system 46) or illumination system 42 by reticlestage 44. Wafer 64 is moved perpendicularly to the optical axis ofprojection optical system 46 by a wafer stage 52. Scanning of reticle 68and wafer 64 generally occurs while reticle 68 and wafer 64 are movingsubstantially synchronously.

Alternatively, photolithography apparatus or exposure apparatus 40 maybe a step-and-repeat type photolithography system that exposes reticle68 while reticle 68 and wafer 64 are stationary, i.e., at asubstantially constant velocity of approximately zero meters per second.In one step and repeat process, wafer 64 is in a substantially constantposition relative to reticle 68 and projection optical system 46 duringthe exposure of an individual field. Subsequently, between consecutiveexposure steps, wafer 64 is consecutively moved by wafer positioningstage 52 perpendicularly to the optical axis of projection opticalsystem 46 and reticle 68 for exposure. Following this process, theimages on reticle 68 may be sequentially exposed onto the fields ofwafer 64 so that the next field of semiconductor wafer 64 is broughtinto position relative to illumination system 42, reticle 68, andprojection optical system 46.

It should be understood that the use of photolithography apparatus orexposure apparatus 40, as described above, is not limited to being usedin a photolithography system for semiconductor manufacturing. Forexample, photolithography apparatus 40 may be used as a part of a liquidcrystal display (LCD) photolithography system that exposes an LCD devicepattern onto a rectangular glass plate or a photolithography system formanufacturing a thin film magnetic head.

The illumination source of illumination system 42 may be g-line (436nanometers (nm)), i-line (365 nm), a KrF excimer laser (248 nm), an ArFexcimer laser (193 nm), and an F₂-type laser (157 nm). Alternatively,illumination system 42 may also use charged particle beams such as x-rayand electron beams. For instance, in the case where an electron beam isused, thermionic emission type lanthanum hexaboride (LaB₆) or tantalum(Ta) may be used as an electron gun. Furthermore, in the case where anelectron beam is used, the structure may be such that either a mask isused or a pattern may be directly formed on a substrate without the useof a mask.

With respect to projection optical system 46, when far ultra-violet rayssuch as an excimer laser is used, glass materials such as quartz andfluorite that transmit far ultra-violet rays is preferably used. Wheneither an F₂-type laser or an x-ray is used, projection optical system46 may be either catadioptric or refractive (a reticle may be of acorresponding reflective type), and when an electron beam is used,electron optics may comprise electron lenses and deflectors. As will beappreciated by those skilled in the art, the optical path for theelectron beams is generally in a vacuum.

In addition, with an exposure device that employs vacuum ultra-violet(VUV) radiation of a wavelength that is approximately 200 nm or lower,use of a catadioptric type optical system may be considered. Examples ofa catadioptric type of optical system include, but are not limited to,those described in Japan Patent Application Disclosure No. 8-171054published in the Official gazette for Laid-Open Patent Applications andits counterpart U.S. Pat. No. 5,668,672, as well as in Japan PatentApplication Disclosure No. 10-20195 and its counterpart U.S. Pat. No.5,835,275, which are all incorporated herein by reference in theirentireties. In these examples, the reflecting optical device may be acatadioptric optical system incorporating a beam splitter and a concavemirror. Japan Patent Application Disclosure (Hei) No. 8-334695 publishedin the Official gazette for Laid-Open Patent Applications and itscounterpart U.S. Pat. No. 5,689,377, as well as Japan Patent ApplicationDisclosure No. 10-3039 and its counterpart U.S. Pat. No. 5,892,117,which are all incorporated herein by reference in their entireties.These examples describe a reflecting-refracting type of optical systemthat incorporates a concave mirror, but without a beam splitter, and mayalso be suitable for use with the present invention.

Further, in photolithography systems, when linear motors (see U.S. Pat.No. 5,623,853 or 5,528,118, which are each incorporated herein byreference in their entireties) are used in a wafer stage or a reticlestage, the linear motors may be either an air levitation type thatemploys air bearings or a magnetic levitation type that uses Lorentzforces or reactance forces. Additionally, the stage may also move alonga guide, or may be a guideless type stage which uses no guide.

Alternatively, a wafer stage or a reticle stage may be driven by aplanar motor which drives a stage through the use of electromagneticforces generated by a magnet unit that has magnets arranged in twodimensions and an armature coil unit that has coil in facing positionsin two dimensions. With this type of drive system, one of the magnetunit or the armature coil unit is connected to the stage, while theother is mounted on the moving plane side of the stage.

Movement of the stages as described above generates reaction forceswhich may affect performance of an overall photolithography system.Reaction forces generated by the wafer (substrate) stage motion may bemechanically released to the floor or ground by use of a frame member asdescribed above, as well as in U.S. Pat. No. 5,528,118 and publishedJapanese Patent Application Disclosure No. 8-166475. Additionally,reaction forces generated by the reticle (mask) stage motion may bemechanically released to the floor (ground) by use of a frame member asdescribed in U.S. Pat. No. 5,874,820 and published Japanese PatentApplication Disclosure No. 8-330224, which are each incorporated hereinby reference in their entireties.

Isolaters such as isolators 54 may generally be associated with anactive vibration isolation system (AVIS). An AVIS generally controlsvibrations associated with forces 112, i.e., vibrational forces, whichare experienced by a stage assembly or, more generally, by aphotolithography machine such as photolithography apparatus 40 whichincludes a stage assembly.

A photolithography system according to the above-described embodiments,e.g., a photolithography apparatus which may include one or more dualforce actuators, may be built by assembling various subsystems in such amanner that prescribed mechanical accuracy, electrical accuracy, andoptical accuracy are maintained. In order to maintain the variousaccuracies, prior to and following assembly, substantially every opticalsystem may be adjusted to achieve its optical accuracy. Similarly,substantially every mechanical system and substantially every electricalsystem may be adjusted to achieve their respective desired mechanicaland electrical accuracies. The process of assembling each subsystem intoa photolithography system includes, but is not limited to, developingmechanical interfaces, electrical circuit wiring connections, and airpressure plumbing connections between each subsystem. There is also aprocess where each subsystem is assembled prior to assembling aphotolithography system from the various subsystems. Once aphotolithography system is assembled using the various subsystems, anoverall adjustment is generally performed to ensure that substantiallyevery desired accuracy is maintained within the overall photolithographysystem. Additionally, it may be desirable to manufacture an exposuresystem in a clean room where the temperature and humidity arecontrolled.

Further, semiconductor devices may be fabricated using systems describedabove, as will be discussed with reference to FIG. 10. The processbegins at step 1301 in which the function and performancecharacteristics of a semiconductor device are designed or otherwisedetermined. Next, in step 1302, a reticle (mask) in which has a patternis designed based upon the design of the semiconductor device. It shouldbe appreciated that in a parallel step 1303, a wafer is made from asilicon material. Making a wafer may include subjecting the wafer to achemical mechanical planarization process that allows a surface of thewafer to be polished. The mask pattern designed in step 1302 is exposedonto the wafer fabricated in step 1303 in step 1304 by aphotolithography system. One process of exposing a mask pattern onto awafer will be described below with respect to FIG. 11. In step 1305, thesemiconductor device is assembled. The assembly of the semiconductordevice generally includes, but is not limited to, wafer dicingprocesses, bonding processes, and packaging processes. Finally, thecompleted device is inspected in step 1306.

FIG. 11 is a process flow diagram which illustrates the steps associatedwith wafer processing in the case of fabricating semiconductor devicesin accordance with an embodiment of the present invention. In step 1311,the surface of a wafer is oxidized. Then, in step 1312 which is achemical vapor deposition (CVD) step, an insulation film may be formedon the wafer surface. Once the insulation film is formed, in step 1313,electrodes are formed on the wafer by vapor deposition. Then, ions maybe implanted in the wafer using substantially any suitable method instep 1314. As will be appreciated by those skilled in the art, steps1311-1314 are generally considered to be preprocessing steps for wafersduring wafer processing. Further, it should be understood thatselections made in each step, e.g., the concentration of variouschemicals to use in forming an insulation film in step 1312, may be madebased upon processing requirements.

At each stage of wafer processing, when preprocessing steps have beencompleted, post-processing steps may be implemented. Duringpost-processing, initially, in step 1315, photoresist is applied to awafer. Then, in step 1316, an exposure device may be used to transferthe circuit pattern of a reticle to a wafer. Transferring the circuitpattern of the reticle of the wafer generally includes scanning areticle scanning stage which may, in one embodiment, include a forcedamper to dampen vibrations.

After the circuit pattern on a reticle is transferred to a wafer, theexposed wafer is developed in step 1317. Once the exposed wafer isdeveloped, parts other than residual photoresist, e.g., the exposedmaterial surface, may be removed by etching. Finally, in step 1319, anyunnecessary photoresist that remains after etching may be removed. Aswill be appreciated by those skilled in the art, multiple circuitpatterns may be formed through the repetition of the preprocessing andpost-processing steps.

Although only a few embodiments of the present invention have beendescribed, it should be understood that the present invention may beembodied in many other specific forms without departing from the spiritor the scope of the present invention. By way of example, a forcecontrol system has generally been described as having three actuators,two of which are arranged to provide substantially the same force whenthe force control system is in use. In general, however, a force controlsystem may include any number of actuators. Additionally, the actuatorsof a force control system may all be substantially individuallycontrolled, and the spacing of the actuators may also vary.

Geometric equations or relationships used to determine the size of acontact area between a polishing pad and a wafer as a function of alocation of the center of pressure of the polishing pad may vary. Thatis, any suitable geometric relationships may be used to determine thecontact area.

Electromagnetic actuators have generally been described as beingsuitable for use as actuators in a force control system. Electromagneticactuators may include, but are not limited to, EI-core and CI-coreactuators. It should be appreciated, however, that substantially anysuitable actuators, electromagnetic or otherwise, may be used as a partof a force control system for a chemical mechanical planarizationpolishing apparatus.

The steps associated with a wafer polishing operation may varydepending, for example, on the overall requirements of a semiconductorfabrication process. Steps may be added, removed, reordered, and changedwithout departing from the spirit or the scope of the present invention.Therefore, the present examples are to be considered as illustrative andnot restrictive, and the invention is not to be limited to the detailsgiven herein, but may be modified within the scope of the appendedclaims.

1. A chemical mechanical planarization polishing apparatus comprising: apolishing pad; a wafer holder, the wafer holder being arranged tosupport a wafer to be polished using the polishing pad, wherein thepolishing pad is arranged to move relative to the wafer holder such thatan area of contact between the wafer holder and the polishing padvaries; and a force control system, the force control system including acontroller and a plurality of actuators arranged to apply forces to thepolishing pad, the controller being arranged to control the forces asthe area of contact varies to substantially maintain a first polishingpressure on the wafer arranged to be supported by the wafer holder. 2.The chemical mechanical planarization polishing apparatus of claim 1wherein the controller is arranged to vary the forces as the area ofcontact varies to substantially maintain the first polishing pressure onthe wafer arranged to be supported by the wafer holder.
 3. The chemicalmechanical planarization apparatus of claim 1 wherein the plurality ofactuators are electromechanical actuators, and controlling the forcesapplied by the plurality of actuators includes controlling currentsprovided to the actuators.
 4. The chemical mechanical planarizationapparatus of claim 1 wherein the controller is further arranged todetermine the forces, the forces being determined based upon a positionassociated with the polishing pad, the first polishing pressure, an airpressure load on the polishing pad, and a distance between a center ofthe polishing pad and a center of gravity associated with the chemicalmechanical planarization apparatus.
 5. The chemical mechanicalplanarization apparatus of claim 4 wherein the area of contact varieswith the position associated with the polishing pad.
 6. The chemicalmechanical planarization apparatus of claim 4 wherein the positionassociated with the polishing pad is a distance between the center ofthe polishing pad and a center of the wafer arranged to be supported onthe wafer holder.
 7. The chemical mechanical planarization apparatus ofclaim 1 wherein the plurality of actuators includes a first actuator, asecond actuator, and a third actuator, the second actuator and the thirdactuator being arranged to each apply a first force to the polishing padwhile the first actuator is arranged to apply a second force to thepolishing pad.
 8. The chemical mechanical planarization apparatus ofclaim 1 wherein the first polishing pressure is a substantially uniformpolishing contact pressure.
 9. A wafer planarized using the chemicalmechanical planarization apparatus of claim
 1. 10. A method forplanarizing a surface of a wafer using a chemical mechanicalplanarization apparatus, the chemical mechanical planarization apparatusincluding a force system, a polishing pad, and a chuck arranged tosupport the wafer substantially in contact with the polishing pad, theforce system including a plurality of actuators which are arranged toapply forces to the polishing pad, the method comprising: polishing thewafer using the polishing pad, wherein polishing the wafer using thepolishing pad includes rotating the wafer while the wafer is in contactwith the polishing pad; determining a current area of contact betweenthe polishing pad and the wafer; and adjusting the forces applied byeach of the plurality of actuators to substantially maintain a firstpolishing pressure on the wafer, wherein the forces are adjusted basedupon the current area of contact.
 11. The method of claim 10 furtherincluding: determining the forces to be applied by each of the pluralityof actuators to substantially maintain the first polishing pressure onthe wafer, wherein determining the forces includes determining a currentposition associated with the polishing pad, identifying the firstpolishing pressure, identifying an air pressure load on the polishingpad, and determining a current distance between a center of thepolishing pad and a center of gravity associated with the chemicalmechanical planarization apparatus.
 12. The method of claim 11 whereinthe current area of contact varies with the current position associatedwith the polishing pad.
 13. The method of claim 10 wherein the pluralityof actuators are a plurality of electromechanical actuators, and theforces applied by each of the plurality of actuators are arranged tosubstantially pull up on edge areas of the polishing pad.
 14. The methodof claim 13 wherein the forces applied by each of the plurality ofactuators are varied by altering currents provided to each of theplurality of actuators.
 15. The method of claim 10 wherein the pluralityof actuators includes a first actuator, a second actuator, and a thirdactuators, the first actuator and the second actuator being arranged tobe controlled substantially together to apply forces of substantiallythe same magnitude.
 16. The method of claim 10 further including:setting parameters associated with the chemical mechanical planarizationapparatus.
 17. The method of claim 16 wherein the chemical mechanicalplanarization apparatus further includes an arm, the arm being arrangedto position the polishing pad, and wherein the parameters include atleast one of the first polishing pressure, a polishing time, a chuckrotating speed, a polishing pad rotating speed, and an arm movingtrajectory.
 18. A force control system suitable for maintainingapproximately a first polishing pressure on a wafer being polished usinga polishing pad of a chemical mechanical planarization apparatus, theforce control system comprising: a controller, the controller beingarranged to determine a first suction force and a second suction forcethat are suitable for enabling a first polishing pressure to be appliedto the wafer while the wafer is being polished; a first actuator, thefirst actuator being arranged to apply the first suction force at afirst area approximately near an edge of the polishing pad tosubstantially pull up on the edge of the polishing pad; and a secondactuator, the second actuator being arranged to apply the second suctionforce at a second area approximately near the edge of the polishing padto substantially pull up on the edge of the polishing pad.
 19. The forcecontrol system of claim 18 wherein the controller is arranged todetermine the first suction force and the second suction force based ona contact area between the wafer being polished and the polishing pad.20. The force control system of claim 19 wherein the controller isfurther arranged to determine the first suction force and the secondsuction force based on the first polishing pressure, a location of acenter of gravity of the chemical mechanical planarization apparatus, anair pressure load applied on the polishing pad, and at least onelocation associated with the polishing pad.
 21. The force control systemof claim 18 further including a third actuator, the third actuator beingarrange to apply the second suction force at a third area approximatelynear the edge of the polishing pad to substantially pull up on the edgeof the polishing pad.